With advancements in material science and engineering, high velocity ion beams have found numerous applications in various fields, such as nanotechnology, metallurgy, etc. In particular, ion beams are used in various techniques, for example surface modification (implantation, etching, ashing, passivation, oxidation), thin film stress relief, oxidizing, nitriding, ion assisted deposition, ion beam insitu precleaning, chemically assisted ion beam etching, reactive ion beam etching, inert ion beam milling and ion beam deposition.
In many applications ions with certain energy are extracted from a plasma source by an extraction mechanism and are subsequently accelerated towards a target for a predefined impact and modification of target characteristics. The extracted ions may also be subjected to additional manipulation and conditioning such as, but not limited to, acceleration of the ions to the desired energy level, deflection, elimination of certain particles from the ion beam necessary for treating the target etc.
Such manipulations and conditioning result in low current density of the ion beam that is steered towards the target. Current density of an ion beam becomes more significant in certain ion implantation applications where a high current density is desired. For instance, in case of high dose (implanted ions) semiconductor manufacturing applications, ion implantation takes place at a faster rate if the ion beam current density is high. As a result of the faster rate of the ion implantation, more semiconductor wafers are manufactured per hour. Therefore, it is desirable to generate ion beams of high current densities.
Ion treatment of large area conductive or non conductive materials, such as three-dimensional mechanical parts for machineries or flat plastics and polymers that may exceed 1 m2, is rather difficult to be performed with conventional ion beam technologies due to the need for long exposure time and/or difficulties in manipulating large area ion beam at low energies (below 1100 eV). Moreover, some materials are heath sensitive so that they cannot be directly immersed in plasma, meaning that a remote treatment with ions extracted from plasma is desired.
Furthermore, a highly focused ion beam is required in techniques such as, but not limited to, very large scale integration processes like nanostructuring, for removing of material on sub 100 nanometer scales, for local deposition of conducting and insulating layers, for high resolution electron imagining, micro-electro-mechanical systems etc. A high current density focused ion beam is also required in material processes, for instance, in milling operations, where materials are required to be reduced from a larger size to a smaller size and/or removed. In such processes, the ion beam is subjected to ion beam current losses during its trajectory towards the target. Thus, in certain applications as above, a high current density ion beam in conjunction with an efficient focusing may be desirable.
Due to simplicity of production and extraction, most technologies are based on surface modification by positive ions. However, treatment of surfaces by energetically negative ions gives the advantage of a reduced surface charging. Negative ion beams of hydrogen are also relevant to controlled fusion.
Existing systems and methods employ various principles for generation of high current density ion beams. Such principles include, for example, surface ionization—cesium; (>10 mA/cm2), charge exchange (μA, He, 20 keV), cold cathode (150 μA, 1-5 keV), electron impact ionization (low energy, 30 μA), hadrons (CERN accelerator), schottky, etc.
Furthermore, other known systems and methods for generating ion beams implement ion extraction mechanisms, where the ions are extracted from a plasma source. Such a plasma source may include, for example, a multicusp plasma (filaments); Ar, H2, (40 mA, f=25 mm), an electron cyclotron resonance plasma (cyclotron applications) 400 μA, (H,He,C,N,O,Ne,Ar,Kr,Xe), RF plasma; max 6 mA/cm2, Ar, O2, duoplasmatron; 20 μA, Ar, SF6, oscillation electron plasma (15 μA, 100-3000 eV), constricted dc glow discharge, laser (5 mA of Pb 18+) etc.
Typically, in an ion extraction system using a plasma source, when an electrode is biased, a plasma sheath (space charge) is formed surrounding the electrode. The plasma sheath, which is a potential structure of space charge, determines to a great extent, the direction of the ion beam extracted from the plasma source. If the sheath is parallel with the extraction electrode, then the ions are extracted perpendicular to this surface and the current density is given by the plasma parameters (plasma density (ne) and electron temperature (Te)). If the sheath has a curvature different from zero, the potential structure within the sheath affects the ion trajectories resulting in focusing or defocusing of the ion beam. Also, an increase in bias (extraction voltage applied at the electrode) has an effect on the plasma sheath and consequently on the ion beam energy and also on the directionality of the ion if the applied bias affect the sheath curvature in the vicinity of the extraction area. The aforementioned factors thus provide some play factors that can be minimized or enhanced with respect to the desired application.
Existing systems and methods describe extraction of high current density ion beam using plasma source. In one embodiment the system employs a hot filament electron bombardment ion generator as the plasma source. A plasma sheath is generated and a conductor-insulator configuration is employed for extraction of ion beam of high current density from the plasma sheath. The system includes an ion source aperture of a small size defined by a focus electrode for minimizing the effect of large extraction voltages on the plasma sheath formed. However, the system is complex to implement and has a plurality of adjustable variables for a given current density of the ion beam.
In practice, any electrode has a finite dimension that inevitably causes a so-called “edge effect” equivalent with a curvature of the sheath structure and correspondingly of the potential distribution within that part of the sheath. Such curved potential can affect the ion trajectories and it is either avoided or used when dealing with ion extraction mechanisms. A conductor-insulator interface is a direct way to produce an edge effect that can influence the directionality of an ion beam. Such alternative—or others including additional biased electrodes that can affect the potential profile within the sheath in the proximity of the extraction orifice and consequently focus or defocus the ions—was used in several applications such as e.g. U.S. Pat. No. 5,825,035 and UK981297. However, neither U.S. Pat. No. 5,825,035 nor UK981297 takes into consideration the exact distribution of the ion current over the surface of the biased electrode interfacing an insulator, the ion current distribution being not only non-uniform over the surface, but being also of a discrete nature, which includes well distinct parts on the electrode that are, for example, not reached by ions or reached with at very high current density resulted by ion focusing.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is provided only to illustrate one exemplary technology area where some embodiments described herein may be practiced.